Method for obtaining an extreme ultraviolet radiation source, radiation source and use in lithography

ABSTRACT

Method for obtaining extreme ultraviolet radiation and a source thereof, application in lithography. 
     According to the invention, at least a solid target ( 28 ) is used, emitting extreme ultraviolet radiation by interaction with a laser beam focussed on a face ( 30 ) of the target. This target is able to emit a portion of the radiation from the opposite face ( 37 ) and this portion is collected and transmitted.

TECHNICAL FIELD

The present invention relates to a method for obtaining extremeultraviolet radiation which is also called “EUV radiation” and to asource thereof.

This is radiation with a wavelength included in the range from 8nanometers to 25 nanometers.

The present invention has many applications, especially in materialscience, in microscopy and most particularly in lithography.

The present invention also relates to a lithography device which usesthe source of EUV radiation, object of the invention.

By using such radiation, the etching pitch of integrated circuits may bereduced and integrated circuits with a very high integration level maybe manufactured.

PRIOR STATE OF THE ART

Let us recall that a lithography device is for insolating a sampleaccording to a determined design (<<pattern>>). This sample generallycomprises a semiconductor substrate on which is coated a layer ofphotosensitive resin (<<photoresist layer>>) which is provided to beinsolated according to the determined pattern.

A lithography device comprises:

-   -   a source of insolation radiation,    -   a mask on which the pattern to be insolated is reproduced with a        magnification factor at least equal to four,    -   a sample support, and    -   optical means which let the radiation be transferred between the        source and the mask on the one hand and between the mask and the        sample on the other hand.

Two techniques for producing an intense EUV radiation are mainly known.Both of them are based on collecting the photons, produced through amicroscopic process of spontaneous emission, by a hot and not very denseplasma which is generated by means of a laser.

The first technique uses a xenon jet irradiated by a YAG laser with apower close to 1 kW. Indeed, when the nature of the gas and theexpansion conditions in vacuo (<<vacuum>>) are well selected, aggregates(<<clusters>>) are naturally generated in the jet, by a multibodyinteraction. These are macroparticles which may contain up to onemillion atoms and exhibit a sufficiently high density (about a tenth ofthe solid's density) in order to absorb the laser beam and thus to heatthe atoms from the surrounding gas which may then emit photons byfluorescence.

The EUV radiation or soft X radiation thereby obtained is collected bysuitable optical means, is spatially shaped through several intermediateoptical means and then it irradiates the mask. The intermediate opticalmeans used are multilayer mirrors which exhibit a high but narrowreflectivity peak (with a transmission bandwidth from 2 to 5% dependingon the considered multilayers) in the neighborhood of the investigatedEUV wavelength (for example, 13.4 nm with alternating Mo and Si layersand 11.2 nm with alternating Mo and Be layers).

The second technique uses the corona of a plasma with a high atomicnumber, obtained by interaction of a laser beam, which is issued from aKrF laser and has an intensity close to 10¹² W/cm², and from a solidtarget of a large thickness (at least 20 μm).

This is schematically illustrated in FIG. 1 where this solid target 2 isto be seen, wherein the laser beam 6 is focussed on a face 4 thereof,via suitable focussing optical means 8. In FIG. 1, the EUV radiation 10generated by the interaction of the focussed laser beam with the targetmaterial is also seen. This radiation is emitted from face 4 called the<<front face>> and is recovered by appropriate optical collecting means12.

In the illustrated example, these optical collecting means 12 arepositioned facing the front face 4, they include an aperture 14 to letthe focussed laser beam pass through and collect the EUV radiation 10 inorder to send it towards other optical means (not illustrated) in orderto use this EUV radiation. The most suitable material for this kind ofsource seems to be rhenium for an emission close to 13.4 nm. Theconversion ratio obtained with this material (ratio between the emittedradiative energy and the incident energy) may even reach 0.85% in a 2%transmission bandwidth around this wavelength of 13.4 nm.

The energy of such a EUV radiation source is however insufficient as thelaser energy, in the case of the quoted experiments, is only of theorder of 1 J to a few joules.

But above all, the photon collecting efficiency is low (of the order of10%) and this finally results in that the yield (usable photons/laserenergy) is too low. Furthermore, the target's expansion is significantso that specific devices must be designed in order to keep the emittedparticles away from the collecting optical means during the interactionof the laser beam with the target.

The aforementioned difficulties result from the nature of the physicalprocess used, i.e. emission by fluorescence from a hot and not verydense medium. Indeed, when a bound electron is excited within an atom ora multicharged ion, by a process involving either a photon (radiativemechanism) or an electron (collision mechanism), this atom or this ionis again found to be in an excited state which is not stable. It thenmay decay by emitting one or several photons.

In order to obtain a photon with a precise wavelength (to within thetransition width), it is therefore sufficient to generate a suitablemulticharged ion wherein energy transitions matching the energy of therequired photon exist. It should be noted that when the photon isemitted through spontaneous emission, it does not have any favoreddirection and an isotropic emission is obtained.

One of the best techniques suitable for generating a large number ofexcited multicharged ions, uses the interaction of a power laser beamwith a high density medium. Actually, when a power laser beam interactswith a solid (or quasi-solid) target, the electromagnetic waveassociated with the laser beam is propagated in the medium up to aso-called cut-off density (which is inversely proportional to λ² where λis the laser's wavelength) and it transfers its energy to this mediumvia several microscopic processes.

Bound electrons are then likely to be extracted from the atoms, to beaccelerated by the electric field generated by the laser and to gainsufficient kinetic energy so as to extract in turn other boundelectrons. Multicharged ions are thereby generated, the temperature ofthe medium rapidly increases until it reaches extreme values (severalhundreds of thousands, even several millions of degrees) and microscopicprocesses leading to emission of photons may occur. In fact under theaction of the laser field, the medium is changed into a plasma formed bymulticharged ions, electrons and photons.

Except for particular density, temperature and/or radiation fieldconditions, the various aforementioned species are not in equilibriumwith one another. This is notably observed in the corona of a plasmawhich corresponds to the expansion area where the laser'selectromagnetic wave is propagated and strongly interacts with themedium. This corona is characterized by low matter density (less than0.001 times the solid's density) and by a high temperature. Thelikelihood that an emitted photon in the corona is reabsorbed therein,is extremely low. This corona is said to be optically thin.

The emitted photons leave the plasma and may then be used for differentpurposes, for example for diagnosing thermodynamic conditions of themedium by spectroscopy or for a lithography.

Let us re-examine the drawbacks of known EUV radiation sources.

These sources pose a problem of efficiency: highly varied thermodynamicconditions (density, temperature, number of free electrons) areencountered in the corona of a plasma generated by a laser, both overtime and space.

The characteristic emission spectrum of radiation from a corona close to10 nm is very complex and consists of a large number of emission linesproduced by atomic emission or from different states of charge. When awell determined line is selected with a very narrow bandwidth (of theorder of 2%), it is seen that a large part of the energy emitted by theplasma as radiation is outside this bandwidth and is therefore lost.

Accordingly, efficiency (produced and used EUV energy/used laser energy)is strongly reduced. Further, <<parasitic>> radiation is emittedisotropically, in particular, in the solid angle for collecting usefulphotons and therefore towards the optical means for collecting thesesphotons.

As for collecting EUV radiation, because photon emission by a hot jet isisotropic, suitable optical collecting means should be provided.Generally, an umbrella-shaped optical collector, obtained byjuxtaposition of elementary optical collectors (generally six of them).In order that its solid angle is maximum, this collector should exhibita large surface area and should be placed as close as possible to theplasma which emits the EUV radiation.

This is materially very difficult (especially in the case of use ofxenon aggregates because of the presence of a nozzle and of a xenonrecovering system) and this also causes problems in terms of thecollector's lifetime and of the achievement of the latter. Thiscollector should therefore be positioned away from the EUV radiationsource, however the collection angle will then be reduced (unless agiant collector is built for which the costs would be prohibitive). Thistherefore results in a loss of efficiency.

The same problems are posed in the case of the use of a solid targetwhere the EUV radiation is emitted by the front face of this target.Moreover, in this case, the plasma corona generated by the laser has avery large expansion velocity (greater than 10⁵ cm/s), even for moderatelaser illumination. Accordingly, the particles of matter are likely tocontaminate and damage the different optics used, with consequently arisk of reducing the reflection properties of these optics and thus thenumber of photons which reach the photosensitive resin layer to beinsolated. Specific devices need to be designed for removing theseparticles or remnants.

Further, as emission by fluorescence from a hot and not very denseplasma does not have a favored direction, specific optical means shouldbe inserted between the collector and the mask in order to spatiallyshape the radiation field. These specific optical means comprisemultilayer mirrors and therefore lead to a loss of photons. They arealso a further cause of cost and difficulties in optical alignment.

A EUV radiation source using a thick solid target, which emits the EUVradiation through the front face, the face receiving the focused laserbeam, thus suffers from different drawbacks, i.e. emission of remnantsand isotropic emission of EUV radiation which therefore has a largeangular divergence. As a result, in particular, a lithography deviceusing such a source is not very efficient.

DESCRIPTION OF THE INVENTION

The object of the present invention is to overcome the above drawbacksby providing a EUV radiation source which is anisotropic. This EUVradiation (for example to be used in a lithography device) is emitted bythe back face of a solid target of suitable thickness on the front faceof which is focused the laser beam.

Such an anisotropic source provides an increase in the useful portion ofthe EUV radiation beam and a simplification of the optical means forcollecting this radiation.

Specifically, the object of the present invention is a method forobtaining extreme ultraviolet radiation, a method according to which atleast a solid target is used, with first and second faces, wherein thistarget is able to emit extreme ultraviolet radiation by interaction witha laser beam, and the laser beam is focussed on the first face of thetarget, a method characterized in that the target contains materialwhich is able to emit the extreme ultraviolet radiation by interactingwith the laser beam, and in that the thickness of the target is in arange from about 0.05 μm to about 5 μm, wherein the target is able toemit a portion of the extreme ultraviolet radiation from the second faceof this target, anisotropically, and in that this portion of extremeultraviolet radiation is collected and transmitted for this portion tobe used.

The object of the invention is also an extreme ultraviolet radiationsource, wherein this source comprises at least a solid target, withfirst and second faces, wherein this target is able to emit extremeultraviolet radiation by interacting with a laser beam focussed on thefirst face of the target, this source is characterized in that thetarget contains material which is able to emit the extreme ultravioletradiation by interacting with the laser beam, and in that the thicknessof the target is included in a range from about 0.05 μm to about 5 μm,wherein the target is able to anisotropically emit a portion of theextreme ultraviolet radiation from the second face of this target,wherein this extreme ultraviolet radiation portion is collected andtransmitted for this portion to be used.

Preferably, the atomic number of the material contained in the targetbelongs to the set of atomic numbers from 28 to 92.

According to a particular embodiment of the source which is the objectof the invention, this source comprises a plurality of targets which arefirmly secured to one another, the source further comprising means formoving this plurality of targets so that these targets receive the laserbeam in succession.

The source may further comprise support means to which the targets arefixed and which are able to let the laser beam pass through in thedirection of these targets, wherein the moving means are provided formoving these support means and thereby the targets.

These support means may be able to absorb radiations emitted by thefirst face of each target receiving the laser beam and to re-emit theseradiations towards this target.

According to a first particular embodiment of the source, object of theinvention, these support means comprise an aperture facing each target,wherein this aperture is delimited by two walls substantially parallelto one another and perpendicular to this target.

According to a second particular embodiment, these support meanscomprise an aperture facing each target, wherein this aperture isdelimited by two walls which run towards the target and away from oneanother.

According to a particular embodiment of the invention, the sourcefurther comprises fixed auxiliary means which are able to let the laserbeam pass through in the direction of the target, to absorb radiationsemitted by the first face of this target and to re-emit these radiationstowards this target.

The object of the present invention is also a lithography devicecomprising:

-   -   a support for a sample which is to be insolated according to a        determined pattern,    -   an extreme ultraviolet source in accordance with the invention,    -   a mask comprising the determined pattern in an enlarged form,    -   optical means for collecting and transmitting, to the mask, the        portion of extreme ultraviolet radiation from the second face of        the target of the source, wherein the mask is thereby able to        provide an image of the pattern in an enlarged form, and    -   optical means for reducing this image and for projecting the        reduced image on the sample.

The sample may comprise a semi-conductor substrate whereon is coated alayer of photosensitive resin which is to be insolated according to thedetermined pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood upon reading thedescription of exemplary embodiments given hereafter, for purelyinformative and non-limiting purposes, with reference to the appendeddrawings wherein:

FIG. 1 is a schematic view of a known EUV radiation source and it hasalready been described,

FIG. 2 is a schematic view of a particular embodiment of the lithographydevice, object of the invention which uses a EUV radiation source inaccordance with the invention,

FIG. 3 is a perspective schematic view of a strip forming a set oftargets which are usable in the invention,

FIGS. 4 and 5 are perspective schematic and partial views of EUVradiation sources according to the invention, and

FIG. 6 is a perspective schematic and partial view of another EUVradiation source in accordance with the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

A plasma generated through the interaction of a solid target and of alaser beam includes several areas. Of course there is the interactionarea which is called <<the corona>> but there is also, in a successiveand simplified way:

-   -   an area called <<conduction area>> where the laser beam does not        penetrate and the evolution of which is controlled by thermal,        electron and radiative conductions, wherein a portion of the        photons emitted by the ions of the corona, is emitted in the        direction of the cold and dense portion of the target, and    -   the absorption and re-emission area where the high energy        photons which come from the corona or from the conduction area,        are absorbed by the dense and cold matter and they thus        contribute to heating this matter and thereby to the emission of        lower energy photons.

The latter form a radiative wave which, in the medium, has a favoredpropagation direction, along the temperature gradient and which, whenthe target is not too thick, may exit the target through the back faceof the latter, the face which is geometrically opposite to the one wherethe laser has interacted. The conversion efficiency at the back face(ratio between the radiative energy, including all wavelengths, to theincident laser energy) may be close to 30%.

Such emission from the back face of the target is characterized by avery different spectral distribution from the one from the front facebecause temperature and density conditions of the areas responsible forthe emission of photons, are very different. The emitted radiationnaturally has an angular distribution, even with a perfectly planetarget: this radiation is not isotropic.

Further, the characteristic expansion velocity of the back face islower, by several orders of magnitude, than that of the front face,whereby the major part of the energy is in the form of radiation.

That is why EUV radiation emitted by the back face of a solid targetwith a suitable thickness on the front face of which the laser beam isfocussed, is used in the present invention. In this way, anisotropic EUVradiation is obtained and the matter remnants are reduced to a minimum.

For generating EUV radiation, the target preferably contains a materialfor which the atomic number Z is such that 28≦Z≦92.

It is possible to mix or associate with this material, other materialswhich are also capable of generating, through interaction with the laserbeam, EUV radiation having the right spectral characteristics.

Furthermore, it may be optionally associated with one or several othermaterials with low atomic numbers, for filtering out parasiticradiation.

The target's thickness, containing the material generating the EUVradiation, or active element, is preferably between 0.1 μm and 5 μm.

Preferably, the target is optimized in order to obtain effectiveemission by the back face, without having a too significant expansion ofthe matter.

The laser's characteristics are also adapted (in particular, theduration and the time shape of the provided light pulses, theirwavelength and intensity) for obtaining the thermodynamic conditionsrequired in the target for optimal EUV conversion at the back face, inthe desired range of wavelengths, which ranges for example from 10nanometers to 20 nanometers.

In FIG. 2, a particular embodiment of the EUV radiation source, objectof the invention, is schematically illustrated in a special applicationto lithography.

Now in this FIG. 2, a lithography device is seen schematically,comprising a support 16 of a semiconductor substrate 18, for example asilicon substrate, on which is coated a layer 20 of photosensitiveresin, to be insolated according to a determined pattern.

In addition to the EUV radiation source 22 in accordance with theinvention, the device comprises:

-   -   a mask 24, comprising the pattern in an enlarged form,    -   optical means 26 for collecting and transmitting, to mask 24,        the portion of EUV radiation provided by the back face of the        solid target 28 which the source includes, wherein mask 24        provides an image of this pattern in an enlarged form, and    -   optical means 29 for reducing this image and for projecting the        reduced image onto the layer 20 of photosensitive resin.

The target is for example made out of a material such as silver, copper,samarium or rhenium and it has low thickness (for example of the orderof 1 μm).

In order to generate the EUV radiation for insolating the photosensitiveresin layer, a pulsed beam 34 emitted by a pulsed laser 35 is focussedon a first face 30 of the target, called <<the front face>>, via opticalfocusing means 32. The target 28 then emits anisotropic EUV radiation 36from its back face 37 which is opposite to the front face 30.

It is specified that source 22, collector 26, mask 24, optical means 29and support 16 bearing the substrate 20 are placed in en enclosure (notshown) where low pressure is applied. The laser beam is sent into thisenclosure through a suitable porthole (not shown).

In the example of FIG. 2, the optical collecting means 26 consist of anoptical collector which is positioned facing the back face 37 of target28, provided for collecting the EUV radiation anisotropically emitted bythis back face, for shaping this radiation and sending it to mask 24.

In the device of FIG. 2, provision of further optical means betweencollector 26 and mask 34 is therefore not required, thereby simplifyingthe optical means of the lithography device.

It is seen that the target with low thickness 28 is fixed by its frontface 30 to a support 38 provided with an aperture 40 for letting thefocussed laser beam 34 pass through so that it reaches this front face.

Practically, as a single laser pulse locally destroys the target withlow thickness, it is not possible to send the laser beam onto the samespot of the target, twice. That is why the support 38 is provided withmoving means (not shown in FIG. 2) which enable the different areas ofthe target to be successively exposed to the focussed laser beam.

This is schematically illustrated in FIG. 3, where a solid target 42with low thickness (for example 1 μm) is seen as a strip, fixed to aflexible support 44 which, for example, is made in plastic and providedwith a longitudinal aperture 46 in order to let the focussed beam 34pass through.

The target-support as a whole forms a flexible composite strip which isunwound from a first spool 48 and is wound onto a second spool 50 whichmay be rotated by suitable means (not shown), so as to move the targetfacing the focussed laser beam including pulses which successively reachdifferent areas of the target. It may then be considered that severaltargets are assembled together.

In an alternative embodiment (not shown), it is further possible to usea flexible strip in plastic as a target support and to fix severaltargets on this support, at regular intervals, wherein an aperture isthen provided in the support facing each target so as to let thefocussed beam pass through.

Preferably, instead of a plastic strip, a strip 52 (FIG. 4) for examplein copper, silver, samarium or rhenium, is used as target support, andable to absorb radiation(s) emitted by the front face of target 42 underthe impact of the focussed beam 34 and to re-emit this (these)radiation(s) in the direction of this target (which is movable with thestrip 52). For example this strip 52 has a thickness of the order of 5μm to 10 μm.

The longitudinal aperture letting the laser beam 34 pass through, whichis focussed on the target, may be delimited by two walls 54 and 56substantially parallel to one another and substantially perpendicular tothe target as is seen in FIG. 4.

However, for a better absorption of the radiation(s) emitted by thefront face of the target and a better re-emission of the latter towardsthe target, it is preferable that both walls delimiting the aperture runtowards the target and away from one another as seen in FIG. 5 whereboth walls are referenced as 55 and 57.

In another example schematically illustrated in FIG. 6, target 42 isfixed to a movable support 44 of the kind which was described inreference to FIG. 3. Moreover, in the example of FIG. 6, the EUVradiation source comprises a part 58 fixed with respect to the focussedlaser beam 34 and positioned facing the front face of the target.

This part comprises an aperture letting the laser beam pass through,which is focussed on this front face of the target and the aperture withwhich this part is provided, flares out towards the target and thuscomprises two walls 60 and 62 inclined with respect of this target,running towards the target and away from one another.

The radiation(s) 64 emitted by the front face of target 42 are thenabsorbed by these walls 60 and 62 and re-emitted in the direction of thefront face of the target.

EUV radiation 36 emitted by the back face of the target is thus moreintense.

Of course, an X ray source using X ray emission from the back face of atarget formed by an aluminium sheet with a thickness of 7 μm and thefront face of which is irradiated by a laser beam with a power densityof 3×10³ W/cm², is known from an article by H. Hirose et al., Prog.Crystal Growth and Charact., vol. 33, 1996, pp. 227–280.

However it should be noted that the method and the source, object of thepresent invention, use a target of low thickness, in the range fromabout 0.05 μm to about 5 μm, wherein this target is preferably made outof a material for which the atomic number Z is much greater than that ofaluminum because Z is preferably greater or equal to 28 (and less orequal to 92).

It is specified that the preferred material for forming the target usedin the present invention is tin for which Z is 50.

Moreover, in the invention, a target with a very thin thickness may beused, less or equal to 1 μm, formed on a plastic substrate (for examplea CH₂ (polyethylene) substrate with a thickness of 1 μm), wherein theback face of this target (preferably in tin)—the face which emits theEUV radiation used—lies on the substrate. It is also possible to form onthe front face of this target, a gold layer with a thickness less than1000 Å (i.e. 100 nm).

Coming back to the aforementioned article, it should be noted that thealuminum target with a thickness of 7 μm cannot be considered for anemission by its back face when its front face irradiated by laserradiation with a maximum power density less than 3×10¹³ W/cm², asmentioned in the article, and this, particularly in the field ofmicrolithography, where the maximum power density considered above forexample, is close to 10¹² W/cm².

It should also be noted what follows:

When the laser interaction occurs on a material with a low atomic numberZ, like aluminum (Z=13), the transfer of laser energy absorbed in thecorona (on the side where the laser interacts: front face) to the coldand dense areas (i.e. to the back face) occurs by thermal electronconduction. Even if the target is relatively thick like the one providedin the aforementioned article, achieving anisotropic emission at theback face is not at all guaranteed.

On the other hand, in the case of high Z material, radiative conductionis what <<controls>> the conditioning of the inside and back of thetarget. Anisotropy which makes the present invention interesting, isdirectly linked to the outgoing of this radiative wave at the back face,thus to the selection of a thickness, for which an optimized value willbe given in what follows.

On the other hand, the temperature and electron density characteristicprofiles in the target irradiated by laser are very different accordingto whether the material has a low or high atomic number and alsoaccording to the target thickness used.

The optimal thickness E₀ for optimizing the conversion ratio X at theback face may be found by using an analytical model. E₀ is related tothe atomic number Z of the target's material, to the atomic mass A ofthis material, to temperature T (°K) in this medium (itself related tothe absorbed laser flux φ_(a) expressed in W/cm²), to the laser'swavelength λ (μm), to the pulse duration Dt (seconds) and to the massdensity ρ(g/cm³) by the following formula:E ₀(cm)=26.22(A/Z)^(0.5) ×T ^(0.5) ×D _(t)/αwithα=ρ×λ²×(1+0.946(A/Z)^(0.5))

Temperatures (°K) is proportional to φ_(a) ^(2/3) and to λ^(4/3).

For low available laser energy (less than 1 J), which is generallyrequired within the scope of applying the invention to lithography,because a very high frequency (greater than 1 kHz) is required forproducing sufficient statistics at the level of the photosensitive resin(thus ensuring that the insolation threshold is reached), and for agiven emissive surface area (set by optimum coupling with the usedoptical system) (for example close to a diameter of 300 μm), theincident laser flux on the target is low. With a nanosecond rate, itdoes not exceed 10¹² W/cm² at 1,06 μm. Furthermore, today, manufacturinglasers with these frequencies based on a 100 ps pulse train ispractically inconceivable.

Under these conditions, the above model gives a value of 30 eV, as themedium temperature which may be attained if all the energy is absorbed.

Under these conditions, for aluminium, the optimal thickness whichoptimizes the conversion rate X at the back face is 0.15 μm, which isvery far from the condition given in the aforementioned article.Furthermore, with a material such as aluminium, of low atomic number,the radiation emitted by the back face of the target does not a prioriexhibit any angular feature: it is substantially isotropic; front faceand back face may therefore be considered as equivalent.

In the case of gold, still under the same conditions, it is found to beless than 0.1 μm.

Referring back to the example given earlier of a tin target, formed on aCH₂ (polyethylene) substrate, the following specifications are given:polyethylene, which may be put on the back face of a thin sheet of tin,and gold which may be put on the front face of this sheet, both serve tolimit the expansion of the emitter material formed by tin before it isheated by the radiative wave, so as to have the photons better<<penetrate>> the area of interest of the target. The polyethylene atthe back face, which is slightly heated, is transparent to the radiationand also limits this expansion and consequently a little of the emissionof matter remnants.

1. A method for obtaining extreme ultraviolet radiation, methodaccording to which at least one solid target is used, said at least onesolid target having first and second faces and being able to emit anextreme ultraviolet radiation by interaction with a laser beam, and thelaser beam is focussed on the first face of the target, characterized inthat the target contains a material which is able to emit the extremeultraviolet radiation by interaction with the laser beam, and in thatthe thickness of the target is in a range from about 0.05 μm to about 5μm, wherein the target is able to emit a portion of the extremeultraviolet radiation from the second face of this targetanisotropically, and in that this portion of extreme ultravioletradiation is collected and transmitted for this portion to be used. 2.An extreme ultraviolet radiation source, comprising at least a solidtarget having first and second faces and being able to emit an extremeultraviolet radiation by interaction with a laser beam focussed on thefirst face of the target, this source being characterized in that thetarget contains a material which is able to emit the extreme ultravioletradiation by interaction with the laser beam, and in that the thicknessof the target is in a range from about 0.05 μm to about 5 μm, whereinthe target is able to anisotropically emit a portion of the extremeultraviolet radiation from the second face of this target and whereinthis portion of extreme ultraviolet radiation is collected andtransmitted for this portion to be used.
 3. A source according to claim2, wherein the atomic number of the material contained in the targetbelongs to the set of atomic numbers from 28 to
 92. 4. A sourceaccording to claim 2, comprising a plurality of targets which areintegral with one another, wherein the source further comprises meansfor moving this plurality of targets so that these targets successivelyreceive the laser beam.
 5. A source according to claim 4, furthercomprising support means to which the targets are fixed and which areable to let the laser beam pass through toward these targets, whereinmoving means are provided for moving these support means and thereby thetargets.
 6. A source according to claim 5, wherein the support means areable to absorb radiations emitted by the first face of each target whichreceives the laser beam and to re-emit these radiations towards thistarget.
 7. A source according to claim 5, wherein the support meanscomprise an aperture facing each target, wherein this aperture isdelimited by two walls substantially parallel to one another andperpendicular to this target.
 8. A source according to claim 5, whereinthe support means comprise an aperture facing each target, wherein thisaperture is delimited by two walls which run towards the target and awayfrom one another.
 9. A source according to claim 2, further comprisingfixed auxiliary means which are able to let the laser beam pass throughtoward the target, to absorb the radiations emitted by the first face ofthis target and to re-emit these radiations towards this target.
 10. Alithography device comprising: a support of a sample to be insolatedaccording to a determined pattern, an extreme ultraviolet radiationsource, according to claim 2, a mask comprising the determined patternin an enlarged form, optical means for collecting and transmitting, tothe mask, the portion of extreme ultraviolet radiation from the secondface of the target of the source, wherein the mask is thus able toprovide an image of the pattern in an enlarged form, and optical meansfor reducing this image and for projecting the reduced image on thesample.
 11. A device according to claim 10, wherein the sample comprisesa semi-conductor substrate whereon is coated a photoresist layer whichis to be insolated according to the determined pattern.